321.306 Roadways laned for traffic.

Whenever any roadway has been divided into three or more clearly marked
lanes for traffic the following rules in addition to all others consistent
herewith shall apply:

A vehicle shall be driven as nearly as practical entirely within a single
lane and shall not be moved from such lane until the driver has first
ascertained that such movement can be made with safety.

Upon a roadway which is divided into three lanes a vehicle shall not be
driven in the center lane except when overtaking and passing another vehicle
where the roadway is clearly visible and such center lane is clear of traffic
within a safe distance, or in preparation for a left turn or where such center
lane is at the time allocated exclusively to traffic moving in the direction
the vehicle is proceeding and is signposted to give notice of such allocation.

Official signs may be erected directing slow-moving traffic to use a
designated lane or allocating specified lanes to traffic moving in the same
direction and drivers of vehicles shall obey the directions of every such
sign.

Vehicles moving in a lane designated for slow-moving traffic shall yield
the right of way to vehicles moving in the same direction in a lane not so
designated when such lanes merge to form a single lane.

A portion of a highway provided with a lane for slow-moving vehicles does
not become a roadway marked for three lanes of traffic.